Secondary batteries, also known as rechargeable batteries, are finding increasing commercial favor in a variety of applications. Some of these batteries comprise a negative electrode which is capable of reversibly electrochemically storing hydrogen. The negative electrode contains an active material which may be a metal alloy comprised of titanium, nickel and usually other materials. Other metallic alloys can also be used as the active material. Suitable alloys are disclosed in U.S. Pat. No. 4,551,400, for example.
The production of these negative electrodes is sometimes difficult because many hydrogen storage active materials are not very ductile, and they are of relatively great or high hardness. Indeed, these alloys can have a Rockwell "C" (R.sub.C) hardness of 45 to 60 or more. Usually the alloy is formed from a melt, and the resulting alloy material must then be crushed or otherwise worked before the material is fabricated into an electrode. The alloys are usually utilized in powdered form in the manufacture of the electrode. This powder takes the form of small ash or flake-like particles which after suitable treatment will pass through a 200 U.S. mesh screen, and thus are smaller than 38 microns in size (200 U.S. mesh screen has interstices of about 75 microns).
Various methods of manufacturing electrode strip have been previously offered, but these methods and their associated equipment cannot be used with the high hardness, flake or ash-like active powdered material involved here. For example, a system for making battery plates is suggested in U.S. Pat. Nos. 3,894,886 and 3,951,688, but that system involves using an electrochemically active thixotropic paste.
Another method of making electrode strip involves feeding a free-flowing silver powder to a moving paper web. Vibrating doctor blades spread the powder on the carrier to a pre-determined thickness. A silver grid structure or mesh is introduced to the powder and carrier. A single rolling mill compresses the grid and powder on the carrier, and then the carrier is withdrawn. The remaining web is then sintered. After the sintered silver strip leaves the sintering furnace, it is cut into strips for use in silver-zinc battery cells.
This system cannot be used with the high hardness active powdered material involved here, because the present powder does not act in the same way as the silver powder and mesh in the silver electrode production line and production equipment. When the high hardness powder used here is compressed on a paper carrier, the powder particles stick to or become embedded in the paper. Web tearing or other web damage can result. In addition, the present invention contemplates depositing a relatively thin layer of flake-like particles on a smooth, hard carrier. It has been found that doctor blades are ill-suited to provide a precisely controlled thickness or depth of powder, because the powder flakes or ash-like particles tend to commingle and build up in front of the blades. A powder layer of irregular thickness and density and occasionally inadequate depth results. Uniformity of powder depth and amount of active material per unit area is necessary to provide a uniform electrode strip. A uniform strip thickness is essential for battery electrodes if finished battery design capacity and performance are to be achieved.
Accordingly, a need exists for a production method and production apparatus capable of making electrode strip from very hard flake-like powder feed stock material. More specifically, a need exists for a method and apparatus to form negative electrode strip material from powdered alloy materials having a Rockwell "C" hardness (R.sub.C) of 45 to 60 or more. As used here, the term "high hardness" will be understood to refer to a Rockwell "C" hardness of 45 to 60 or more.